1. Field of the Invention
This invention relates to a photosensitive material for electrophotography and more particularly, to a photosensitive material for electrophotography which comprises a support, a photoconductive layer and an electrically insulating layer, which can be suitably employed in an electrophotographic method for the formation of an electrostatic latent image and which has high sensitivity, panchromatic sensitivity and electric charging ability.
2. Description of the Prior Art
In known electrophotographic methods, it is a common practice to reproduce an image by providing a photoconductive layer on an electrically conductive support, electrically charging the photoconductive layer surface using a corona discharger or a like device, exposing the surface to light in an imagewise pattern to permit the electric charge in the exposed area to discharge whereby an electric charge remains in an imagewise pattern to form an electrostatic latent image on the surface, developing the latent image with colored charged particles, transferring the developed and colored particle image to paper or another support, and fixing the transferred image. In this process, amorphous selenium, zinc oxide and like materials are generally used as materials for the photoconductive layer.
Although the process is very simple in operation since the electrostatic latent image can be formed in the two steps of charging and exposing, several problems exist in the method in that the photoconductive layer is susceptible to damage since the photoconductive layer is the outermost layer, with the possibility of an attendant unattractive image, and in that the photoconductive material is harmful to the human body due to its toxicity and is thus disadvantageous from the viewpoint of safety.
U.S. Pat. No. 3,041,167 describes another type of electrophotographic method in which a photosensitive element provided with an electrically insulating layer on the outer surface thereof is used for formation of an electrostatic latent image. In this process, an electrostatic latent image is formed by the steps of electrically charging to a given polarity an electrically insulating layer surface of a photosensitive element of a three-layer structure comprising an electrically conductive support, a photoconductive layer and an electrically insulating layer, uniformly exposing the element to light to produce an electric charge with a polarity opposite that of the first charging at the interface between the photoconductive layer and the electrically insulating layer, subjecting the electrically insulating layer surface to a second charging in a polarity opposite that of the first charging for neutralization of the surface electric charge, and exposing the element to light in an imagewise pattern to form an electrostatic latent image at the interface between the electrically insulating layer and the photoconductive layer. As will be understood from the above, the electrically insulating layer must be transparent to radiation such as light or X-rays to which the photoconductive layer is sensitive.
The electrophotographic process using a photosensitive element of the type described immediately above has a number of advantages. One advantage is that the surface of the photoconductive layer itself is not damaged since the steps of developing, transferring, cleaning, etc., are carried out on the surface of the electrically insulating layer. Another advantage is that the photosensitive element is usable over a long period of time by an appropriate selection of an electrically insulating material with high hardness. A further advantage is that no special care needs to be taken with regard to the toxicity of the photoconductive material since the photoconductive layer is covered with the electrically insulating layer.
However, the process described in U.S. Pat No. 3,041,167 has a disadvantage. That is, in the known electrophotographic process, the charging is effected by depositing ions from a corona discharger on the surface of a photosensitive material. While, in the process of the U.S. Pat. No. 3,041,167, the charging is conducted by transferring charge carries from the electrically conductive support to the interface between the photoconductive layer and the electrically insulating layer, so that the charge carries tend to dissipate unless there is a satisfactory charge retention in the neighborhood of the surface of the photoconductive layer. Accordingly, a high charging current is disadvantageously required in the first charging step in order to create a quite high electric field and to cause a large amount of charge carriers to be produced. In addition, such a high charging current undesirably involves the generation of ozone. This disadvantage is emphasized especially when a highly sensitive material such as a Se alloy containing As and/or Te is employed as the photoconductive layer.
In order to improve the charging ability, an attempt has been made to use a material with high electrical resistance as a surface layer of the photoconductive layer. However, such an attempt is disadvantageous not only in reducing the sensitivity of the photosensitive material, but also in increasing the residual potential when the photosensitive material is repeatedly employed.